1. Field of the Invention
The present invention relates to an image forming apparatus and a method of correcting image concentration for an image forming apparatus.
2. Description of the Background Art
Generally, image forming apparatuses may need adjustment of image density (or concentration) at a given timing. For example, in an image forming apparatus of JP-2008-83252-A, a control method to reduce time required for re-adjustment of image density during an image forming operation is disclosed. When the re-adjustment timing of image density comes during the image forming operation, patches are formed and the concentration of patches are detected to determine whether re-adjustment of image density is required based on the extent of difference between the detected patch concentration and a target.
Further, for example, JP-2007-279523-A discloses an image forming apparatus that can suppress the downtime occurrence and can efficiently conduct processes such as an image forming operation. In JP-2007-279523-A, the image forming apparatus such as a copier includes a photoconductor drum, which forms toner images to be transferred to recording sheets or patch images to be read by a concentration sensor, and an intermediate transfer belt to transfer images from the photoconductor drum at a transfer position. The intermediate transfer belt is disposed with a position sensor facing the belt and a control unit that obtains spectrum data of the intermediate transfer belt detected by the position sensor, and sets a phase address on the intermediate transfer belt as a reference position.
In tandem type image forming apparatuses such as laser beam printers, a plurality of image forming units is used to superimpose toner images on an intermediate transfer belt to form a color image. However, the properties of the toner are affected by environmental changes, by which toner concentration when toner images are transferred to a paper may change, and thereby a stable color image cannot be obtained.
In light of such situation, a concentration correction process is conducted, in general, in which concentration correction patterns are formed for each color, a detector such as a toner mark (TM) sensor detects transferred toner concentration, and toner concentration is corrected to a target concentration.
The TM sensor may be of two types. One type of sensor has a light-receiving unit that can measure only regular reflection light. The other type of sensor has a light receiving unit that can measure both regular reflection light and diffuse reflection light. In low-end laser beam printers, the number of TM sensors that can measure both regular reflection light and diffuse reflection light is reduced to a minimum to reduce cost.
The concentration correction patterns are required to be provided under TM sensors that can measure both regular reflection light and diffuse reflection light. Further, to use uniform property for light emitting and light receiving of the TM sensor, the concentration correction patterns are usually disposed directly below a single TM sensor. In such a case, as the intermediate transfer belt rotates, the concentration correction patterns ultimately contact a given portion of a transfer roller and toner concentrates at that portion. After all the concentration correction patterns pass over the transfer roller, the roller must be cleaned to remove the toner. Further, even if the concentration is corrected once, the concentration may deviate from the target concentration due to environmental condition changes or simply over time. Therefore concentration correction may need to be conducted periodically. Even then, however, the concentration correction patterns formed on the intermediate transfer belt adhere to the transfer roller, and the transfer roller must be cleaned to remove the toner.
The transfer roller can be cleaned by applying plus and minus bias voltage alternately to the transfer roller, by which toner can be scattered to the intermediate transfer belt. Accordingly, the cleaning time becomes longer in proportion to the concentration (or level of contamination) of the most contaminated portion on the transfer roller. A printing operation cannot be conducted when the transfer roller is undergoing cleaning, which constitutes significant downtime for a user and decreases productivity.